In selecting materials for corrosive sour oil field environments, the materials of choice must be reliable and cost-effective. Materials have to meet criteria for corrosion resistance and mechanical properties in service environments for the required service life. Both precipitation hardened nickel-base alloys and cold-worked solid solution nickel-base alloys offer many advantages such as high-strength, toughness and excellent corrosion resistance. INCOLOY alloy 825 (UNS N08825) and INCOLOY alloy 028 (UNS N08028) are among the primary solid solution Corrosion Resistant Alloys (CRA's) currently used in the cold worked condition for Oil Country Tubular Goods (OCTG's) in sour gas wells. One of the primary CRA machining quality age-hardened bar product used with alloys 825 and 028 OCTG's for wellhead and subsurface completions of gas wells is INCOLOY alloy 925 (UNS N09925).

Traditionally, in the selection of OCTG's for sour gas service, CRA's are screened first by their pitting resistance equivalent number (PREN) and then by environmental cracking data generated in sour brine environments. The theory is that pitting occurs first, which provides a stress-riser for initiation of anodic chloride stress corrosion cracking (SCC). While alloy 028 has a somewhat higher PREN than alloy 825, alloy 825 has a significantly higher nickel content. H.R.Copson originally reported the effect of nickel content on chloride SCC resistance of austenitic alloys in 1959. Slow strain rate (SSR) tests conducted in severe sour brine environments showed that the higher nickel content of alloy 825 results in better stress corrosion cracking resistance than that exhibited by alloy 028. This suggests that in some cases for austenitic alloys, the nickel content of the CRA may be more important than the PREN in OCTG selection. As alloy 925 is similar in alloy Ni, Cr, Mo and Fe content to alloy 825, the alloys exhibits excellent resistance to SCC and hydrogen embrittlement.

This paper presents guideline tables and graphs for cold-worked austenitic alloys 825 and 028 and age-hardened alloy 925. Based on an extensive literature review of laboratory test and field data, the alloys have demonstrated corrosion resistance up to 230ºC depending on the chloride concentration, and H₂S content.